Schematic for a hybrid 12AU7 amp?

[mark2]

Short version: I'd like to build a moderately loud (ballpark 12-50W) hybrid amp using 12AU7 in the preamp, but am a little out of my depth, and having trouble finding a pre-made schematic.  Any pointers?

Longer version:
I have a couple of 12AU7 tubes, and was thinking of building a Tube Cricket

This uses the lm386, which I have a handful of, but this is only <.5W and really quiet.

I see there are lots of schematics and kits for TDA2030/2040/2050 power amps. I started with the idea to simply swap out the "power amp" part of the schematic, but am very confused about how to do it. e.g. different input voltages.

I then explored trying to modify the Tiny Giant, and remove its preamp as directed here, but am again confused because of different voltage requirements, and general lack of confidence.

Ideally I'd like to find a good, cheap to build, schematic so I can make a head for my 1x12 (8ohm). Any advice or leads would be really appreciated!

[tubegeek]

In general, I think "I have x tube so I'm going to build something with x tube" is not always the greatest approach. That said, I've built stuff that way and gotten good results so maybe you will too.

Tubes are - again generally - intended to be powered by higher voltages than transistors and ICs (which contain transistors anyway.) So most "real" tube circuits would use something over 150 volts with a 12A(anything)7* and when you get down below 80V or so you are talking "starved" operation. Then the tube will also require a heater supply of either 12.6 or 6.3 V which should be DC if it's doing the first voltage amplification job at the input. And the chip amp will be happier with something over 12V for max power out.

So you do indeed have some contradictory power supply requirements.

If I was doing this, I'd build a switch-mode power supply  (look up "Nixie SMPS") and run it off of maybe 18V DC. That would boost the 18V to 200+ volts to run my tube, with plenty of current available for multiple 12A(anything)7's.

Then I'd also use the 18V for the chip amp, and add a 12V regulator off of 18V to provide the DC for the heaters.

A little tricky. Probably why you're not seeing a ton of ready-made options.

* there are several tubes in this family, 12AX7, 12AU7, 12AY7, 12AT7. All are dual triodes that can be operated off of 6.3 OR 12.6 volt heater supplies. They have the same pinouts but the triode characteristics of each are somewhat different.

[mark2]

Thanks this is very helpful. Out of curiosity, do you think this is how the new breed of lunchbox amps are doing it (e.g. Joyo Bantamp, Orange micro) ?

Either way, it sounds like it might be a little trickier than I'm prepared to handle.

Point taken on the "I have X so I'll build Y"... This all was rooted in having most of the stuff to build the Tube Cricket, which sounds good in demo vids, but hoping for more volume. Clearly it's is running "starved"  (12v DC). If I wanted to take this schematic and simply build a more powerful power amp section (i.e. not lm386), I'd still need some kind of switch-mode power supply, yeah?

[vigilante397]

do you think this is how the new breed of lunchbox amps are doing it (e.g. Joyo Bantamp, Orange micro) ?

Yes, that's exactly how they do it.

If I wanted to take this schematic and simply build a more powerful power amp section (i.e. not lm386), I'd still need some kind of switch-mode power supply, yeah?

Not necessarily. Depends on how much power you want, what chip you've found that can do it, and what voltage that chip wants to run at. There are chips out there (especially class-D) that can get plenty of power out of an 18V supply, you'll just need to make sure your supply has enough current handling to give the chip what it needs.

[mark2]

Thanks, that's good to know.

There are chips out there (especially class-D) that can get plenty of power out of an 18V supply, you'll just need to make sure your supply has enough current handling to give the chip what it needs.

What do you think of taking that Tube Cricket schematic (http://beavisaudio.com/projects/TubeCricket/img/TubeCricket_Schematic_Rev_1_05_November.gif),  feeding it with a 12V 3A dc power supply, and just replacing the power amp with this TDA2030 board?

This should be a really cheap and easy way to make that Cricket jump from 0.5W to 18W, no?

Details on that TDA2030 board for posterity in case that link breaks:

TDA2030A Module DIY Kit Parts 6-12V Single Power Supply Audio Amplifier Board Module

Description:

TDA2030A audio amplifier circuit, often using V-5-pin single in-line plastic package structure. The integrated circuit is widely used in car stereo receiver tape recorders, medium-power audio equipment, with small size, large output power, distortion and other characteristics. And has an internal protection circuit.
Feature:
1. onboard TDA2030A audio power amplifier chip
2. mono 18W amplifier circuit design
3.on-board speaker wiring blocks
4.onboard 10K adjustable resistance, you can adjust the volume of amplification
5. Onboard power indicator
6. the chip has been the main pin leads, you can directly enter the audio signal
7. the working voltage: 6 ~ 12V
8.The peak output current is 3A

I'll continue doing more research to see if I can work this out, before falling back to the initial plan to just build a Tube Cricket.

[vigilante397]

That would almost work, but there's a couple things you need to think about, the main one being that 3A for the power section isn't your only current sink for the system. The tube section will pull 150mA just for the heaters, so it's safe to call that another 0.2A, so a total of 3.2A for the system.

Advice from an engineer: always over-design your power supply. I design circuits for a living, and my boss requires that all power supplies be designed to handle double what they need in the worst-case scenario. For example, if you have a preamp pulling 200mA and a power amp pulling 3A, your power supply would need to be able to handle 6.4A. This is overkill for a lot of things, but to be safe I would recommend at least a 5A supply. You never want to be operating at or even near the limit of your supply.

So with a decently beefy power supply, that should absolutely be a cheap and easy way to turn the Tube Cricket into a bigger amp.

[mark2]

So with a decently beefy power supply, that should absolutely be a cheap and easy way to turn the Tube Cricket into a bigger amp.

Awesome, thanks. Easy enough. I'm sure I can find a 4A supply.

Excited to try this without a big investment or risk.

[mark2]

An update if anyone is curious, or others stumble on this and want to try a similar hybrid amp...

After I get my parts in, I'll still build the tube cricket and see how it sounds, but found this Frog tube preamp that seems 100x better.  It really sounds fantastic in the couple videos I watched, and has some fun (for building, tinkering, learning AND playing) extra controls like clipping, EQ, bright switch.

It has an internal SMPS (185VDC) so it's not running the tube starved. I'll feed it, and the power amp with a 12V laptop power supply.

[cab42]

An update if anyone is curious, or others stumble on this and want to try a similar hybrid amp...

After I get my parts in, I'll still build the tube cricket and see how it sounds, but found this Frog tube preamp that seems 100x better.  It really sounds fantastic in the couple videos I watched, and has some fun (for building, tinkering, learning AND playing) extra controls like clipping, EQ, bright switch.

It has an internal SMPS (185VDC) so it's not running the tube starved. I'll feed it, and the power amp with a 12V laptop power supply.

The Frog preamp looks great but you can also contact Nathan (Vigilante397) a couple of posts above. He makes a similar preamp using a SMPS. A couple of weeks ago I received a great looking pcb from him. I believe you could use a 12AU7 for that. I bought some sub mini tubes for it.

https://www.diystompboxes.com/smfforum/index.php?topic=122111.0

I am going to try it as a standalone preamp, into different power amps. I have an old Guyatone hybrid amp (solid state pre/tube power) that I would like to try as an all tube amp. Last year ago I bought a 80's Cube 60 for bass. It could be fun to run the preamp into the return of the effects loop. I even have a half finished 2050 power amp somewhere.

[vigilante397]

The Frog preamp looks great but you can also contact Nathan (Vigilante397) a couple of posts above. He makes a similar preamp using a SMPS. A couple of weeks ago I received a great looking pcb from him. I believe you could use a 12AU7 for that. I bought some sub mini tubes for it.

https://www.diystompboxes.com/smfforum/index.php?topic=122111.0

I am going to try it as a standalone preamp, into different power amps. I have an old Guyatone hybrid amp (solid state pre/tube power) that I would like to try as an all tube amp. Last year ago I bought a 80's Cube 60 for bass. It could be fun to run the preamp into the return of the effects loop. I even have a half finished 2050 power amp somewhere.

Thanks for the plug Carsten My Particle Accelerator is based on the Alembic F2B preamp like the Frog preamp, the noticeable difference being, like you mentioned, I like submini tubes. You could absolutely use a 12AU7 if you wanted, but it would be a lot lower gain. A 6N21B (my favorite) or 12AX7 will sound closer to the original preamp. It sounds great for guitar or bass, and I've run it direct into a PA or it can be run straight into a power amp.

[mark2]

I should have thought to ask here first - I already ordered that board.  If it doesn't work out or I want to experiment with a variation I'll definitely reach out. Thanks!

[vigilante397]

No worries, PCB sales aren't a huge priority for me. And I've heard good things about the Frog board (almost bought one myself when I was still figuring things out), I'm sure it will work out. But on the off-chance it doesn't I'm very familiar with the circuit, so at the very least I could help with debugging

[tubegeek]

A 6N21B (my favorite)

Are you getting the Russian subminis from the ebay sources or do you have someplace better?

[vigilante397]

Are you getting the Russian subminis from the ebay sources or do you have someplace better?

eBay. I have a handful of 6N21B designs I was selling regularly, so about 8 months ago I just bought 100 6N21B I got them pretty cheap (I want to say around $120 for the lot), and now I don't have to worry about sourcing for a while.

[printer2]

do you think this is how the new breed of lunchbox amps are doing it (e.g. Joyo Bantamp, Orange micro) ?

Yes, that's exactly how they do it.

Not in the case of the Joyo Meteor. Came across their RFI testing documentation and the caps on the tube board are only 25V. They have two 220uF caps and a 47uf cap. Not sure what they are doing, there is an IC by the tube. If they were doing any stacking of voltages I would think the caps would be the same value, with the two 220u it would be 40V, ad another 20 with the 47u. While a little low for a 12AX7 60-70V is conceivable. Not that this has any bearing on what people have been doing using a HV supply. I just thought it might be of interest to others, I was wondering how they were using the tube in their amps. Still curious what they are doing.

https://fccid.io/2AIN7BANTAMP/Internal-Photos/Internal-Photos-3736803


Looked up the Orange Micro Dark, the plate seems to run on low voltage.

[vigilante397]

Not in the case of the Joyo Meteor. Came across their RFI testing documentation and the caps on the tube board are only 25V. They have two 220uF caps and a 47uf cap. Not sure what they are doing, there is an IC by the tube. If they were doing any stacking of voltages I would think the caps would be the same value, with the two 220u it would be 40V, ad another 20 with the 47u. While a little low for a 12AX7 60-70V is conceivable. Not that this has any bearing on what people have been doing using a HV supply. I just thought it might be of interest to others, I was wondering how they were using the tube in their amps. Still curious what they are doing.

https://fccid.io/2AIN7BANTAMP/Internal-Photos/Internal-Photos-3736803


Looked up the Orange Micro Dark, the plate seems to run on low voltage.

Interesting, thanks for the info. When I was first setting up my recording rig I bought a couple 12AX7 mic preamps for cheap, only to find they ran 24V on the plates. Was there a tube? Yes. Did the audio go through it? Also yes. Did it improve the tonal quality of the cound? Not even a little bit.

I'm trying to remember which lunchbox amp I was looking at that did this, apparently it wasn't the ones mentioned. I remember it having an odd power supply, 17V input? Something like that.

[mark2]

One more very rudimentary question:

Should I run every ground back to a lug screwed into the chasis? Or do need separate grounds for anything?

[vigilante397]

So you definitely don't need separate grounds, it should be a common ground. Running all the grounds back to a single point on the chassis (star grounding) is a great idea, especially for high gain, but I admittedly don't usually do it. Use a common ground for all ground connections, but in connecting board ground to chassis ground, just do what makes sense to you and doesn't make wiring a nightmare.

[printer2]

Not in the case of the Joyo Meteor. Came across their RFI testing documentation and the caps on the tube board are only 25V. They have two 220uF caps and a 47uf cap. Not sure what they are doing, there is an IC by the tube. If they were doing any stacking of voltages I would think the caps would be the same value, with the two 220u it would be 40V, ad another 20 with the 47u. While a little low for a 12AX7 60-70V is conceivable. Not that this has any bearing on what people have been doing using a HV supply. I just thought it might be of interest to others, I was wondering how they were using the tube in their amps. Still curious what they are doing.

https://fccid.io/2AIN7BANTAMP/Internal-Photos/Internal-Photos-3736803


Looked up the Orange Micro Dark, the plate seems to run on low voltage.

Interesting, thanks for the info. When I was first setting up my recording rig I bought a couple 12AX7 mic preamps for cheap, only to find they ran 24V on the plates. Was there a tube? Yes. Did the audio go through it? Also yes. Did it improve the tonal quality of the cound? Not even a little bit.

I'm trying to remember which lunchbox amp I was looking at that did this, apparently it wasn't the ones mentioned. I remember it having an odd power supply, 17V input? Something like that.

They used the Meteor in the test but the whole lineup of Joyo mini amps will have the same deal. When a company goes through EMI testing they do not want to go through the process with every piece of equipment so they keep modules as standard as possible and tell the powers that be functionally nothing is different. I think the supply was 17-18V for them all. That is how they get the power amp pushing out the watts, the voltage what it is due to the laptop market, no reason to develop your own stuff when an off the shelf item is cheaper.